A permanent magnet electric machine is a synchronous machine whose rotor magnetic field is created by means of permanent magnets fitted to the rotor pack. Since the rotor pack rotates at high speeds when the machine is running, the fitting of the magnets must be strong enough to withstand the centrifugal forces, as well as the magnetic forces appearing between the magnets themselves. While rotor packs employed in electric machines without permanent magnets are formed by steel sheets joined together by means of pressure, this type of rotor pack is not employed in permanent magnet machines because of the difficulty of making screwed holes in the steel sheets for fitting the magnets. For this reason, rotor packs used in permanent magnet electric machines are usually formed by a single solid piece of steel to which the magnets are more easily fitted.
There are mainly two ways of fitting the magnets to the solid piece of steel: embedded or surface mounted. Embedding the magnets in the rotor pack is usually very complex and expensive, partly because the magnetic forces appearing between the magnets make the embedding operation extremely difficult. On the other hand, while fitting the magnets on the surface of the rotor pack is easier to do, the magnetic and centrifugal forces appearing within the magnets when the machine is in use are often too strong, this solution being thus limited to slow machines.
Additionally, generally speaking all pieces forming the rotor must be placed accurately in their location in the rotor pack, since an inaccurate fitting of the pieces may cause the machine to be noisy. Specifically, an inadequate position of the magnets may cause a drop in the performance of the machine, as well as high levels of noise and vibration.
On the other hand, rare earth permanent magnets may suffer demagnetization if overheated, some consequences being a drop in performance due to an increase in air gap surface losses, a cooling system for the rotor, such as cooling ducts, being thus necessary.